UPS and FedEx said they are grounding their fleets of McDonnell Douglas MD-11 cargo planes "out of an abundance of caution" following a deadly crash at the UPS global aviation hub in Kentucky.

The crash on Tuesday at UPS Worldport in Louisville killed 14 people, including the three pilots on the MD-11 that was headed for Honolulu. National Transportation Safety Board official Todd Inman said Thursday that the crashed plane was a 1991 McDonnell Douglas 2 that had been "altered" into a McDonnell Douglas MD-11 Freighter.

MD-11 aircrafts make up about 9% of of the UPS airline fleet and 4% of the FedEx fleet, the companies said.

"We made this decision proactively at the recommendation of the aircraft manufacturer," a UPS statement said late Friday. "Nothing is more important to us than the safety of our employees and the communities we serve."

FedEx said in an email that it will be grounding the aircrafts while it conducts "a thorough safety review based on the recommendation of the manufacturer."

The planes were built by McDonnell Douglas, but the company merged with Boeing in 1997, making Boeing the manufacturer of record for the aircraft. Boeing told CBS News in a statement that it made the recommendation to ground the aircraft "out of an abundance of caution."

Boeing's Safety Review Board assessed all available information about the crash and determined that further engineering analysis was needed, a company spokesperson told CBS News via email. The manufacturer recommended pausing fleet operations while that analysis was conducted. Inman said Thursday that the crashed plane was currently being "handled" by Boeing.

Boeing announced in 1998 that it would be phasing out its MD-11 jetliner production, with final deliveries due in 2000. There are about 70 MD-11 planes in operation today. The planes are only used to ferry cargo. FedEx and UPS each have about two dozen MD-11s, and a third airline, Western Global, has 16. Twelve of those planes were already put in storage, according to the aviation analytics firm Cirium. CBS News reached out to Western Global for comment on the grounding.

Flight records suggest the UPS MD-11 that crashed underwent maintenance while it was on the ground in San Antonio, Texas, for more than a month until mid-October. It is not clear what work was done.

The UPS cargo plane was nearly airborne Tuesday when a bell sounded in the cockpit, Inman said during a briefing Friday. For the next 25 seconds, the bell rang and the pilots tried to control the aircraft as it barely lifted off the runway, its left wing ablaze and missing an engine, and then plowed into the ground in a spectacular fireball. The plane was carrying about 255,000 pounds of jet fuel, as well as up to 20,000 packages.

The cockpit voice recorder captured the bell, which sounded about 37 seconds after the crew called for takeoff thrust, Inman said. There are different types of alarms with varying meanings, he said, and investigators haven't determined why the bell rang, though they know the left wing was burning and the engine on that side had detached.

Inman said it would be months before a transcript of the cockpit recording is made public as part of that investigation process.

Jeff Guzzetti, a former federal crash investigator, said the bell likely was signaling the engine fire.

"It occurred at a point in the takeoff where they were likely past their decision speed to abort the takeoff," Guzzetti told The Associated Press after Inman's news conference. "They were likely past their critical decision speed to remain on the runway and stop safely. … They'll need to thoroughly investigate the options the crew may or may not have had."

Dramatic video captured the aircraft crashing into businesses and erupting in a fireball. Footage from phones, cars and security cameras has given investigators evidence of what happened from many different angles. Witnesses recalled chaos on the ground.

"It was explosion after explosion after explosion, so you just didn't know when it was going to stop," Georgie Dow, the chief financial officer of an auto parts business that was struck by the plane, told CBS News. "It was so hot ... You took a step back because it was like heat in your face. There was no going to help."

The UPS package handling facility in Louisville is the company's largest. The hub employs more than 20,000 people in the region, handles 300 flights daily and sorts more than 400,000 packages an hour.

UPS Worldport operations resumed Wednesday night with its Next Day Air, or night sort, operation, spokesperson Jim Mayer said.

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